Epoxy resins have conventionally been widely used as heat-resistant resins. In order to improve a flexibility and impact resistance of epoxy resins, there are attempts to directly modify epoxy resins themselves by various modification techniques such as rubber modification and silicone modification. However, such modification techniques are defective in that although an improvement in flexibility, impact resistance, and the like is achieved, there is a problem that a heat resistance deteriorates.
As an expedient for improving flexibility and impact resistance without deterioration of heat resistance, a technique of blending an epoxy resin with an aromatic thermoplastic resin has been proposed as described in, for example, Journal of the Adhesion Society of Japan (Nippon Setchaku Gakkai Shi), 27 (9), 364 (1991). According to this technique, the two resins are dissolved in a good solvent for both resins to obtain a composition in a solution form and this composition is dried by heating to remove the solvent, thereby obtaining a cured product.
The above technique, however, has a problem that since the composition usually contains a heat activatable hardener, a curing reaction proceeds gradually in the stage of heat-drying for solvent removal and, hence, it is difficult to completely remove the solvent contained in the composition and part of the solvent remains in the final cured product. Further, there is a fear that the composition may have poor storage stability because it should be stored in a solution state. In addition, the proposed technique has still another problem that since the composition is in a solution state in which the epoxy resin and the aromatic thermoplastic resin are in a compatibilized state, the composition, when formulated to have a low solvent content and hence a high solid content, comes to have an exceedingly high viscosity and poor coating operability.
On the other hand, as a heat-curing adhesive which can first undergo pre-gelation for provisional bonding and then be heat-cured, an adhesive comprising an epoxy resin as a base polymer and, added thereto, a poly(vinyl chloride) plastisol is disclosed in, for example, West Germany Offenlegungschrift No. 2,002,880. Further, adhesives obtained by adding halogen-free thermoplastic resins, such as poly(vinyl butyral), poly(vinyl formal), poly(meth)acrylates, poly(vinyl acetate), polyamides, polyacrylonitrile, cellulose acetate, ethyl cellulose, and nitrocellulose, to epoxy resins are disclosed in JP-A-56-82865, JP-A-62-252491, JP-B-2-48192, and JP-B-2-48193. (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.)
When used, for example, for bonding a material to another, these adhesives have an advantage that since the adhesive applied undergoes pre-gelation upon heating for a short period of time and the materials to be bonded are hence brought into a provisionally bonded state, the materials in such a state can be subjected to processings such as bending, cutting, degreasing, and pickling without causing falling or scattering of the adhesive or fouling of the working area or treating liquids by the adhesive.
However, of the above-described adhesives, the adhesive containing poly(vinyl chloride) has a drawback that there are cases that under high-temperature conditions during curing, spot welding, and other operations, a gas is generated due to decomposition of the poly(vinyl chloride) and this gas corrodes steel plates present nearby. The adhesives containing halogen-free thermoplastic resins are defective in that there are cases that bonding strength becomes low as the amount of the thermoplastic resin added increases, or properties such as high-temperature bonding strength and water-immersion bonding strength deteriorate according to the nature of the thermoplastic resin added.